Being new to the 986 and not familiar with all the 986 ling, what is "cat-less"?

This set of pipes is designed to replace the secondary cats on 2000 to 2004 986's. They are not monitored by O2 sensors, so they can be removed. This saves weight and possibly gives better exhaust flow. But being that you have a 1999 model you don't have to worry about that. As far as I know you should have only one set of cats.